Comparison of the immune response to Shope fibroma virus induced tumor in adult rabbits which exhibit regression is being compared to that of neonatal rabbit which experience progressive tumor growth. The kinetics of development of cytotoxic lymphocytes in newborn rabbits will be determined. Other parameters of immune reactivity both specific and non-specific will be tested to determine if there is a significant immune defect in neonatal rabbits. Finally, attempts will be made to modify the course of tumor growth in neonatal rabbits by transfer of cells and products from adult "regressor" donors.